This invention relates to a Hartley oscillator and in particular Hartley oscillator capable of exactly maintaining its oscillation frequency and oscillation amplitude constant.
A conventional typical Hartley oscillator includes a basic oscillation circuit 1 as shown in FIG. 1. The basic oscillation circuit 1 includes an active element, for example, an npn transistor 2, tank circuit 3 and coupling capacitor C.sub.2. The tank circuit 3 includes a series circuit of inductive impedances Z.sub.1 and Z.sub.2, and a capacitive impedance Z.sub.3 connected in parallel with the series circuit. The junction of the impedances Z.sub.1 and Z.sub.2 is connected to the emitter of the transistor 2. The junction of the impedances Z.sub.2 and Z.sub.3 are grounded. The coupling capacitor C.sub.2 is connected between the base of the transistor 2 and the junction of the impedances Z.sub.1 and Z.sub.3.
A resistor R.sub.3 is connected between the collector of the transistor 2 and a positive pole terminal 4 of a direct current powder source so as to suppress an abnormal oscillation current which flows into the transistor 2. A capacitor C.sub.1 for lowering an AC impedance has one end connected to the junction between the terminal 4 and the resistor R.sub.3. The other end of the capacitor C.sub.1 is grounded. A voltage on the junction of series-connected resistors R.sub.1 and R.sub.2 coupled between the DC terminal 4 and ground is applied through a resistor R to the control electrode of the transistor 2.
If the constant of each part is so set as to satisfy the oscillation frequency requirements and oscillation output amplitude requirements, it is possible to obtain a Hartley oscillator. In the above-mentioned arrangement, however, when the various constants (for example, a current amplification constant) of the transistor are varied during oscillation (they generally tend to be varied), an emitter current is also varied. In consequence the oscillation frequency and amplitude output of the tank circuit 3 are also varied. In order to maintain the oscillation frequency and oscillation output of the oscillator constant it is generally required that electric current, i.e. emitter direct current of the transistor 2, flowing into the tank circuit 3 be maintained constant. It is known that in order to maintain the emitter current constant a resistor, not shown, for suppressing a variation of emitter current is connected between the emitter of the transistor and the tank circuit. Such a means is provided at the sacrifice of oscillation output.
It is accordingly the object of this invention to provide a Hartley oscillator capable of obtaining an oscillation output having a predetermined oscillation frequency and oscillation amplitude.